Multi-channel navigation receivers encoded by pseudo-random codes (PR-codes) are widely used together with global navigation satellite systems, such as GPS, GLN (“GLONASS”) and some others. These GNSS systems are capable of determining a location of the receiver, determining its speed vector and coordinating the receiver clock scale with the system time. The navigation receiver receives signals from a few satellites at the same time. In GPS, each satellite transmits signals using two bands: L1 and L2. Within the L1 band, the carrier of the radio signal is modulated by the pseudo-random (PR) clear acquisition (C/A) code and a pseudo-random precision (P) code. In the L2 band the same P-code is used on a different carrier frequency. GPS satellite signals differ by PR-code structure. In the L1 band, the codes are inversely modulated by binary symbols containing information. The clock rate of the information symbols is 50 Hz.
Similar principles are embodied in receivers of different GNSS, with some adjustments in code structure, carrier frequency, methods of dividing satellite channels, and so on
There are coupled tracking systems in satellite channels of the navigation receiver: a phase lock loop (PLL) 102 to track carrier frequency and a delay lock loop (DLL) 104, used to track the modulating PR code. The DLL generates reference codes and measures time delays that are further transformed in pseudo-ranges. Carrier phases and pseudo-ranges are used for measuring phase and code coordinates respectively, as shown in FIG. 1.
A typical tracking circuit is a cascade connection of single units through which control signals are transmitted. FIG. 2 shows a general structure of a PLL circuit. The DLL is built in a similar manner.
An input signal from a satellite is fed to the signal processing unit in PLL where it is processed with reference carriers and reference codes. A reference carrier u(t) is generated by a numerical control oscillator (NCO) 204, reference codes Π(t) come from the DLL. As a result of the processing, an error signal Z is generated which goes through the next unit (a loop filter 206) and is then fed to control the NCO, thereby closing the circuit.
Tracking systems in navigation receivers are implemented by digital devices, hence control signals in the loops are discretely transmitted at an interval which is called an adjustment period.
Processing in the signal processing units includes generating correlation signals. A correlation signal is a result of accumulating multiplication products of the received signal (passed through input and filtering units and frequency converter in the receiver) multiplied by a reference carrier and a reference code. The reference carrier corresponds to the carrier of the received signal for the given satellite, the reference code corresponds to the PR code of the same satellite. Units that multiply and accumulate are called reset correlators, and the corresponding process is called cross-correlation of two signals. A few reset correlators are assigned for each satellite different from each other in reference signals. Accumulation in digital correlators is implemented by adding digital samples which follow at a certain time interval called epoch.
There are many types of channel configuration for such a receiver. A typical configuration normally includes a few correlators. FIG. 3 shows a scheme of correlators, where there are multiplication operators (X) and reset accumulators Σ↓.
The first correlator calculates an in-phase correlation signal I. This signal can be generated if the input signal is correlated with in-phase reference carrier u1(t)=sin(ωt), the phase of which corresponds to the carrier phase of the input signal and the first reference code Π1. In this correlator, the reference code is a PR-code replica modulating the input signal. Due to their correlation, the in-phase correlation signal I is generated, which is used to demodulate binary information symbols; in different units, this signal is used as an auxiliary normalization signal.
If there is an error during phase correction of the in-phase reference carrier phase co and error of time delay of the reference code ε, then the signal I can be represented as:I=μκUsR0(ε)cos φ,  (1)
where R0(ε) is the cross-correlation function of PR code (passed the receiver filter) and the first reference code Π1, which is a locally-generated copy of the input PR code.
κUs is the value proportional to the amplitude of the input signal.
μ—is the designation of the information binary symbols (μ=±1)
The second correlator calculates the quadrature correlation signal Q. This signal is generated when quadrature reference carrier u2(t)=cos(wt) is present in the correlator, i.e., its phase is shifted by π/2 from the input carrier, and reference code Π1 is the same as in the first path. Their cross-correlation gives the quadrature correlation signal Q, which is used to generate the error signal in the PLL
The signal Q is as follows:Q=μκUsR0(ε)sin φ  (2)
The third path calculates a differential in-phase correlation signal dI intended for controlling the DLL. To generate this signal, the in-phase reference carrier u1(t) (that is in-phase with the input carrier) is employed, and reference code Π2 represent a sequence of short pulses—strobes—which are set at the moments of changing chip signs of the input PR-code. The polarity of the strobes matches the sign of the chip following the next strobe. Each strobe is generated in the form of a group of several rectangular-shaped pulses with different duration (in particular, a simple strobe shaped as a single rectangular pulse is used).
The dI signal is:dI=μκUsΔR0(ε)cos φ,  (3)
where ΔR0(ε) is the cross-correlation function of the modulating PR-code (after the receiver filter) and the reference code consisting of a sequence of short strobes.
Some navigation receiver types have a fourth path where a differential quadrature correlation signal dQ is computed. (See, for example, High-precision positioning apparatuses based on global navigation satellite system signals, Vol. 1, Edited by Zhodzishsky M. I. Moscow, MAI-PRINT, 2010, page 230). To get this signal, quadrature reference carrier and reference code of strobes (as in the third path) are used, but pulse groups can be differently shaped depending on the purpose of the given correlation signal. As a result, the correlation signal dQ is given bydQ=μκUsΔR1(ε)sin φ  (4)
where ΔR1(s) is different from ΔR0(ε) in strobe shapes and time positions in the reference code.
Time shifts of reference codes in the first, second, and fourth paths are tightly coupled with the adjusted DLL shift of the reference code for the third path.
In the signal processing units, correlation signals are converted into error signals, then filtered and used for controlling reference oscillators.
In real conditions, PLLs and DLLs measure phases and delays with errors caused by external disturbances: additive interference (including intrinsic noise of the receiver), fluctuations during radio wave propagation, instability of oscillator frequency etc, as well as dynamic disturbances due to receiver movement. Each external disturbance brings a certain type of errors and needs special measures to be suppressed. One of the most essential additive interference is signals reflected from the ground or local objects. The so-called multipath errors caused by reflected signals can substantially reduce accuracy of both code and carrier phase measurements.
A number of methods are applied to fighting against multipath errors in DLL and PLL. Those based on improving channel structures and not relating to any specific model of the reflected signal can be singled out. Methods of suppressing reflected signals described in U.S. Pat. No. 5,901,183, U.S. Pat. No. 5,953,367, U.S. Pat. No. 6,272,189 and U.S. Pat. No. 6,493,378 are based on applying different shapes of pulse grouping in strobes designed for reference codes.
Final accuracy of navigational measurements is defined by a combination of different disturbances. When one estimates a specific suppression technique for one interference type, he needs to consider a possibility of increasing errors of some other types. Different disturbances can be most dangerous for different scenarios and the consumer needs to choose a receiver or operation mode that is the best for coping with the most dangerous errors in a particular environment.